ISIJ International, Vol. 50 (2010), No. 9, pp. 1265–1269 Supercooling and Solidification Behavior of Phase Change Material LinLin WEI1) and Kenichi OHSASA2) 1) Formerly Graduate Student, Hokkaido University. Now at Nippon Metal Industry Co., Ltd., Hama-cho, Hekinan 447-8610 Japan. 2) Department of Materials Science and Engineering, Akita University, 1-1 Tegata Gakuenmachi, Akita 010-8502 Japan. (Received on January 6, 2010; accepted on June 7, 2010) Supercooling and subsequent solidification behavior of phase change materials (PCM) of sodium acetate trihydrate (SAT) and erythritol were studied by using a thermal analysis technique. A molten SAT specimen easily supercooled below 0°C and the observed maximum degree of supercoolimg was 89°C. Three changes due to phase transformation were observed in the cooling curve of the SAT. First change corre- sponds to the precipitation of sodium acetate (CH3COONa), and second change is the recalescence due to the solidification of supercooled SAT. It was considered that third change corresponds to the solid state transformation of SAT. From the maximum degree of the supercooling of SAT, the solid/liquid interfacial energy of SAT was evaluated as 5.56ϫ10Ϫ2 J/m2. The observed maximum degree of the supercooling of erythritol was 91°C. An experiment to induce the nucleation in supercooled liquid of SAT was carried out and the effectiveness of the combination of the ultrasonic irradiation and the addition of nucleation catalysts was demonstrated. KEY WORDS: phase change material; sodium acetate trihydrate; erythrtol; supercooling; solidification; nucleation. mal storage device. 1. Introduction Recently, the concept of a supercooled thermal energy Recently, latent heat thermal storage technology attracted storage (super-TES) system is proposed.1–3) In the system, a much attention for the storage of the waste energy evolved supercooled liquid of a PCM is kept at room temperature at factories such as ironworks. In a latent heat thermal stor- for a long time, and stored thermal energy is released at an age system, thermal energy is stored by using Phase arbitrary moment by solidifying the supercooled liquid of Change Material (PCM) where a large amount of heat is the PCM. In order to develop the super-TES system, con- saved in the form of transformation latent heat. Trans-Heat trol of the supercooling characteristic of a PCM must be system is an application of PCM, in which industrial waste required. A technique for the inducement of nucleation in heat is stored in a PCM and delivered to a wide area being the supercooled liquid of a PCM at an arbitrary moment away spatially from a heat source. This technology will be will also be required for the development of the super-TES expected for contributing to the reduction in CO2 emission system. In order to control the supercooling and subsequent by reducing combustion of fossil fuel. nucleation behavior of the PCMs, the understanding of the Sodium acetate trihydrate (SAT) (CH3COONa·3H2O) is supercooling characteristics of the PCMs is important. a PCM with melting point of 58°C and used as a relatively The aims of this study are 1) to investigate the supercool- low temperature heat source. Erythritol (HOCH2[CH(OH)]2 ing and subsequent solidification behavior of the SAT and CH2OH) is a PCM with melting point of 119°C and erythritol based on the thermal analysis method and 2) to expected to store the waste energy from heat sources with examine the method for the inducement of the nucleation in relatively higher temperature over 100°C. A feature of these supercooled liquid of the SAT for the purpose of the devel- PCMs is a supercooling characteristic. In these PCMs, opment of the super-TES system. solidification does not occur at the melting point and the PCMs easily become supercooled state for a relatively wide 2. Experimental Method temperature range. To start the solidification of the super- cooled PCMs, nucleation inducement procedures are often 2.1. Materials and Thermal Analysis needed such as the addition of nucleation catalysts, applica- Sodium acetate trihydrate (SAT) (CH3COONa·3H2O) tion of mechanical vibration and ultrasonic irradiation. with purity of 99% and erythritol (HOCH2[CH(OH)]2 Hence, the supercooling characteristic of PCMs was con- CH2OH) with purity of 98% were used as target materials. sidered to be a bothersome feature for using PCM as a ther- Specimens of 10 mg in mass were used for thermal analyses 1265 © 2010 ISIJ ISIJ International, Vol. 50 (2010), No. 9 Fig. 1. Schematic view of experimental apparatus. using TG-DTA (Thermo-Gravimetry Differential Thermal Fig. 2. Thermal analysis curve of sodium acetate trihydrate Analysis) and DSC (differential scanning calorimetry) (SAT) of 20g in mass. under Ar atmosphere. Because of the limitation of working temperature ranges in TG-DTA and DSC, thermal analysis using a furnace with a working temperature range from Ϫ50 to 300°C was also carried out for specimens with 20–30 g in mass. The specimens were set in a glass beaker and the upper surface of the specimen was covered with an oil for the prevention of the decomposition of the specimen as shown in Fig. 1(a). The thermal analysis was carried out in air with heating and cooling rates of 3°C/min. Fig. 3. XRD pattern of solidified sodium acetate trihydrate 2.2. Inducement of Nucleation in Supercooled Liquid (SAT). A SAT specimen of 20g in mass was set in a glass beaker and heated to 100°C, then melted specimen was cooled to room temperature in air. The glass beaker with the specimen was set in an ultrasonic scrubber and ultra- sonic irradiation was applied to the specimen as shown in Fig. 1(b). Ultrasonic irradiation experiment for the speci- men with the addition of starch or metasilicate nonahydrate (Na2SiO3·9H2O), that were known as a nucleation catalyst for SAT, was also carried out to examine the effect of the combination of the ultrasonic irradiation and the nucleation Fig. 4. Thermal analysis curve of solidified sodium acetate trihy- catalyst on the nucleation behavior of SAT. Solidification drate (SAT) specimen in a temperature range below 0°C. behavior of the specimen was monitored by measuring the change in temperature of the specimen. change of the recalescence corresponds to the solidification of supercooled SAT. The third change has not been identi- 3. Results and Discussion fied. To clarify the third phase transformation, heating and 3.1. Supercooling and Solidification Behavior of SAT cooling procedures of a specimen after the second transfor- From the thermal analyses of SAT using TG-DTA and mation was carried out. Observed thermal analysis curve is DSC, it was confirmed that the covered oil prevents SAT shown in Fig. 4. In the heating and cooling stages, changes from the decomposition and the melting point of the SAT is in curve due to phase transformation were observed at al- 58°C from the endothermic peak. due to melting. However, most same temperatures. From this result, it can be consid- solidification did not occurred during the cooling stage of ered that the third change in the curve corresponds to the molten SAT until room temperature. solid state transformation of the SAT. Since this unknown To examine the supercooling behavior of molten SAT in low temperature phase disappears at room temperature, the a temperature range below room temperature, a thermal XRD analysis, which was done at room temperature, could analysis of a specimen of 20 g in mass was carried out not detect this phase. From the viewpoint of the use as a using the furnace with a working temperature range from PCM, most heat energy is stored and released at the melt- Ϫ50 to 300°C. Figure 2 shows a thermal analysis curve ing and solidification stage, and the latent heat energy of of the SAT specimen. Three changes due to phase transfor- the low temperature solid state transformation can be mation were observed in the cooling stage. First change regarded as negligible. occured at 66°C, second change was the recalescence occurred at Ϫ22°C and third one occured at Ϫ17°C. 3.2. Factors Influencing on Supercooling Behavior of Figure 3 shows the result of an XRD analysis of the solidi- SAT fied specimen shown in the Fig. 2. Sodium acetate Figure 5 shows the effect of the superheat of molten (CH3COONa) and sodium acetate trihydrate (SAT) (CH3 SAT on the degree of the supercooling. A SAT specimen of COONa·3H2O) were identified. From this result, it can be 20 g in mass was melted in a glass beaker and kept at cer- considered that the first change corresponds to the precipi- tain temperatures for 30 min and then cooled with cooling tation of sodium acetate (CH3COONa), and the second rate of 3°C/min. From the Fig. 5, a tendency can be seen © 2010 ISIJ 1266 ISIJ International, Vol. 50 (2010), No. 9 Fig. 6. Effect of the holding time of superheated molten SAT on Fig. 5. Effect of the superheat of molten SAT on the degree of the degree of supercooling. the supercooling. Table 1. Comparison of material properties between sodium acetate trihydrate (SAT) and succinonitirile (SCN). that the degree of supercooling increases with increase in mens solidified below 0°C in the experiment. Obviously superheat of the molten SAT. Figure 6 shows the effect of calculated homogeneous nucleation temperature is the holding time of the molten state of SAT on the super- incorrect and this result shows that the S/L interfacial cooling behavior. A SAT specimens of 20g in mass was energy of succionoitrile is inadequate for applying to SAT. melted in the glass beakers and kept at 110°C for given In the present study, an assumption was made; the periods and then cooled with a cooling rate of 3°C/min.